The present invention relates to a copper-based, functionally graded alloy having uniform composition and diameter and continuously or stepwise changing properties such as hardness, modulus elongation, etc. and a method for producing such an alloy, and use of such an alloy in guide wires, catheters, etc.
Functionally graded alloys are materials having continuously or stepwise changing properties such as hardness, elasticity, thermal conductivity, electric conductivity, etc. without gradient in size given by mechanical working such as cutting, etc. or chemical treatments such as etching, etc. Functionally graded materials developed so far are mostly such two-component composites as SiC/C, ZrO/W, TiC/Ni, ZrO/Ni, etc., which have gradually changing mixing ratios.
Conventional functionally graded materials having gradually changing mixing ratios have been produced by mixing different material powders at gradually changing mixing ratios to prepare a plurality of mixed powder sheets having gradually changing mixing ratios, laminating the mixed powder sheets along the gradually changing mixing ratios, compacting and sintering them. For example, Japanese Patent Laid-Open No. 5-278158 discloses a functionally graded, binary metal material produced by laminating and sintering W powder and Mo powder at a gradually changing mixing ratio.
However, the functionally graded materials produced by such a method cannot be rolled or drawn, and they can be formed to desired shapes only by cutting. Thus, they are not only very expensive but also cannot be formed into complicated shapes. Accordingly, the conventional functionally graded materials are used mainly in highly expensive applications, such as spacecraft, nuclear power generators, etc. It is thus highly desired to develop less expensive and easy-to-form functionally graded materials.
Also, alloys having shape recovery properties and superelasticity are widely used in various applications such as guide wires, catheters, etc. To introduce the catheter into the blood vessel and place it at a desired site in the blood vessel, a guide wire for guiding the catheter is first introduced into the desired site in the blood vessel, and the catheter is guided to the desired site in the blood vessel along the guide wire. Because human blood vessels are winding and branching differently depending on individuals, guide wires having high introduction operability and torque conveyance are required to insert the guide wires without damaging the blood vessel walls.
For this purpose, the guide wire is composed of a core wire comprising a tip end portion which is made soft by reducing its diameter, and a body portion which is relatively rigid, and a coating layer formed on the core wire, the coating layer being made of synthetic resins which do not cause any damage to the human body, such as polyamides, thermoplastic polyurethanes, fluoroplastics, etc.
The guide wire is usually constituted by a coil-shaped metal wire made of stainless steel, carbon steel, etc. However, wires of such materials are easily bent, superelastic metals such as Ni--Ti alloys, etc. are used for the core wires of the guide wires (Japanese Patent Publication No. 2-24549).
However, because superelastic Ni--Ti alloys lack rigidity, though they are sufficiently soft. Therefore, they are not well inserted into the blood vessel, sometimes making it difficult to place them at a desired place in the blood vessel.
Also, because Ni--Ti alloys are relatively poor in cold working, they are not easily formed into thin wires suitable for guide wires, etc. With respect to the gradient of properties by heat treatment, it is difficult to provide the guide wire with such a gradient as to control the torque conveyance of the guide wire.
The same is true of catheters made of Ni--Ti alloys. The Ni--Ti alloy catheters are not well inserted into the blood vessel. Also, Ni--Ti alloys are not easily formed into thin wires or pipes. Further, the Ni--Ti alloys are poor in weldability and adhesion, posing problems when combined with other materials.